1. Field of the Invention
The present invention relates generally to the construction and use of vascular catheters, and more particularly to a vascular catheter which combines ultrasonic imaging and balloon dilatation capabilities.
Percutaneous transluminal angioplasty (PCTA), commonly referred to as balloon angioplasty, was first used successfully by Andreas Gruntzig about ten years ago and has enjoyed widespread use and success since that time. Despite its success, there are patient risks involved with balloon angioplasty. In particular, expansion within the blood vessel caused by inflation of the dilatation balloon can cause damage to the blood vessel wall and, after the procedure is complete, loose material which results from tearing of the plaque or atheroma can move into the blood vessel lumen, causing abrupt reclosure. Either event can require immediate surgical intervention. Even in angioplasty procedures which are free from adverse events, a restenosis rate of approximately 30% within six months has been observed.
It is believed that some or all of these problems may be avoided by intraluminal imaging of the treated blood vessel before, during, and after the angioplasty procedure takes place. In particular, ultrasonic imaging allows the determination of the nature of the stenotic material (e.g., the extent of calcification) as well as providing a two-dimensional profile of the stenosed region which can reveal the boundary between the stenotic material and the blood vessel. With such information, the extent of dilatation required can be more accurately forecast, lessening the chance of injury to the blood vessel. Moreover, after treatment, it is possible to examine the blood vessel wall to determine if loose stenotic material is likely to become dislodged and result in abrupt closure of the blood vessel. If such a situation exists, additional treatment can be provided and/or the patient may be observed more closely.
At least in part to provide such advantages, ultrasonic imaging catheters have been developed. See, e.g., U.S. Pat. Nos. 4,794,931; 4,576,177; and 3,938,502, the disclosures of which are described hereinbelow and are incorporated herein by reference. By using such imaging catheters together with separate angioplasty catheters, it is possible to image before and after the angioplasty procedure takes place. The use of separate imaging and angioplasty catheters, however, is problematic since it requires catheter exchange procedures which are both costly and time consuming. Additionally, the repeated insertion and removal of the catheters may exacerbate the injury to the blood vessel and may actually dislodge compressed stenotic material, resulting in reclosure of the blood vessel.
Thus, it would be desirable to combine both ultrasonic imaging and balloon dilatation capabilities on a single catheter which would allow both imaging and dilatation to be accomplished without need to exchange catheters.
U.S. Pat. No. 4,841,977, discussed hereinbelow, discloses such a catheter which combines balloon dilatation and ultrasonic imaging capabilities. A phased-array ultrasonic transducer is located within a dilatation balloon, and the patent describes that imaging may be accomplished using the ultrasonic transducer even while the balloon dilatation takes place. While that may be case, the device described in the '977 patent suffers from a particular problem.
As the ultrasonic transducer is located within the dilatation balloon, the environment surrounding the transducer will be determined by the inflation medium which is utilized. The patent suggests that either saline (which is a good ultrasonic imaging medium) or contrast medium (which is a good fluoroscopic imaging medium) may be used. It is possible, however, to use only one medium at a time and a choice must be made between having an adequate ultrasonic medium and a medium which allows the balloon to be observed by fluoroscopy during the dilatation procedure. The need to make such a choice is an unfortunate compromise in the design of the combined imaging and dilatation catheter of the '977 patent. An additional drawback to the design described in the '977 patent is the increased device diameter which results from the presence of the uninflated balloon over the distal imaging housing. Such increased diameter limits the ability of the catheter to cross particularly tight lesions which may be present in the blood vessel being treated.
For these reasons, it would be desirable to provide combined ultrasonic imaging and dilatation catheters which can use a suitable contrast medium for balloon inflation while maintaining the ultrasonic transducer in an environment optimized for imaging. It would be particularly desirable to provide vascular catheters having reduced diameters where the balloon and ultrasonic transducer are axially spaced-apart and where different media may be introduced to the region surrounding each of the balloon and the ultrasonic transducer.
2. Description of the Background Art
U.S. Pat. No. 4,794,931, describes a combined atherectomy-ultrasonic imaging catheter where a cross-sectional vascular image is obtained substantially at the location where a stenosed region is severed by a rotating blade. Although a balloon is located next to the blade and the imaging transducer, that balloon is intended for positioning of the blade and not for dilatation. U.S. Pat. No. 4,841,977, describes an angioplasty catheter having a phased-array ultrasonic imaging transducer located within a dilatation balloon. U.S. Pat. No. 4,494,549, describes an endoscope having an ultrasonic imaging transducer located within a balloon at its distal end. The ultrasonic signal from the transducer is reflected outwardly by a rotating inclined surface. Other ultrasonic imaging catheters are described in U.S. Pat. Nos. 3,938,502, and 4,576,177.